Mechanical Engineering Design-for-Reliability

This paper presents a design-for-reliability approach for mechanical design. Reliability analysis in part design, indeed the very definition of reliability, has been focused towards the electronic and digital disciplines since the emergence of reliability engineering in the late 1940’s. That focus dictates that parts fail in time; that all parts have a constant failure rate, and that part failure is modeled by the exponential mass density function. This paper presents current research that proposes that reliability in mechanical design is not characterized by ‘best practices’ reliability analyses. One premise investigated is that time does not cause failure in mechanical design; only failure mechanisms do. Mechanical parts experience wear-out and fatigue, unlike electronic and digital parts. Mechanical design analysis for part design investigates material strength properties required to survive failure mechanisms induced by part operation and by part exposure to external failure mechanisms. Such failure mechanisms include physical loads, thermal loads, and reactivity/corrosion. Each failure mechanism acting on a mechanical part induces one or more part failure modes, and each part failure mode has one or more failure effects on the part and the upper design configurations in which the part is integrated. The second premise investigated is that mechanical part failure is modeled by the Weibull mass density function in terms of stress, not time. A reliability math model for tensile strength in composite materials is presented to illustrate the two premises.